The present invention relates to a plant for generating an atmosphere intended for at least one furnace for the heat treatment of metals and among other components incorporating an alcohol of the methanol type, i.e. one having the property of decomposing at the temperature of the furnace by cracking of its molecules to yield carbon monoxide and hydrogen. The plant includes feed devices arranged for simultaneously delivering said alcohol under controlled flow in the liquid phase, and at least one other component of said atmosphere in the gaseous phase, and distribution means in communication with said feed devices and with the furnace. Hereinafter such plant will be designated "plant of the kind described".
Atmospheres of this nature are commonly used in the treatment of metals and are applied more particularly for carrying out the steel treatment processes, for example heating prior to tempering, annealing and hardening (carburizing or carbonitriding). A treatment process of this kind is described in French Patent specification No. 73.45,946 and more particularly in its second patent of addition application filed on the Jan. 15, 1979 under the French Pat. No. 79.00,872. French Patent specification No. 73.45,946 corresponds to U.S. Pat. No. 4,035,203 granted July 12, 1977, and French second patent of addition No. 79.00,872 corresponds to U.S. patent application Ser. No. 107,785 filed Dec. 28, 1979. The process makes use of an atmosphere within the furnace which essentially comprises: a carrier gas consisting at least partially of industrially pure nitrogen; a hydrocarbon; hydrogen and carbon monoxide, these last two components originating from methanol decomposition under heat. A process of this nature consequently presupposes the generation of an atmosphere which incorporates methanol prior to the feeding of the atmosphere into the furnace.
French Patent specification No. 1,189,033 had moreover already disclosed a plant for generating an atmosphere partially formed by alcohol, in particular by methanol, arranged for insertion into a heat treatment furnace. However, this plant differed from that forming the object of the aforesaid addition in that it was arranged to produce an alcohol-town gas mixture. The mixture was produced from a constant-level tank itself supplied from a primary tank by means of a pump controlled by a float system.
The application of the process described in the aforesaid French patent of addition application and the different essential conditions of the present application, in particular the need to exclude any trace of oxygen, steam and carbon dioxide in the furnace, necessitated a complete redesign of the plant for generating the atmosphere intended to be fed into the said furnace.
As a matter of fact, the application of atmospheres based on alcohol and nitrogen raises a particular number of problems. One of these problems arises because it is appropriate to provide the feed to the furnace at a constant and strictly controlled rate of delivery, from two ingredients in two physically different states: the one being gaseous (nitrogen) and the other liquid (alcohol). The technique, known per se, for supplying the alcohol from a tank pressurised by means of compressed air has the disadvantage of causing an occulusion of air in the alcohol, and the gas bubbles forming within the liquid mass cause a diphase flow which interfers in the rate of flow measurement. Furthermore, these bubbles increase the risk of feeding oxygen and other impurities into the furnace.
Another problem arises because the alcohol cracking operation should yield solely carbon monoxide and hydrogen (excluding carbon dioxide gas, steam and methane), that is to say that this cracking operation should occur in accordance with the reaction CH.sub.3 OH.fwdarw.CO+2H.sub.2 in the case of methanol. It is known that this reaction occurs within a particular temperature range of from 700.degree. to 1150.degree. C. for methanol. The vapourisation and cracking of alcohols being endothermic actions, it is of importance that no cold spots should be established within the furnace during infeed of the nitrogen-alcohol mixture, which would lead to a cracking operation generating the aforesaid undesirable products, in particular gaseous carbon dioxide and steam.
It is also appropriate to ensure that the solid impurities which may be present in the alcohol cannot clog the elements for infeed of the nitrogen-alcohol mixture into the furnace.
Thus, it is an object of the invention to provide a solution for the aforesaid problems and to improve plants of the kind described and constituting the prior art.